Electroacoustic devices have included electronic audio noise cancelling devices for the attenuation of broadband noise at the ear. Such devices are either passive devices or active devices. Passive devices have generally comprised rigid cups lined with acoustically absorbent material. The cups are edged with cushions to enclose the ears in substantially air tight cavities lined with the acoustically absorbent material.
Active devices for noise cancellation have included: microphones, amplifiers and loud speakers to measure noises in local areas relatively distant from the sources and to produce equal amplitude and opposite phase acoustic signals to cancel out the sound in the areas. Such known arrangements are prone to produce interference patterns which even increase the noise intensity in other locations.
In one noise cancelling apparatus an array of independent sound cancellation units is arranged over a vibrating noise generating surface. Each unit includes an arrangement of acoustic transducers positioned adjacent the surface to obtain an electrical average of the local acoustic noise generated by a predetermined zone of the surface. The electrical sum average is changed in phase and gain by an active filter whose output drives an acoustic projector also positioned adjacent the surface. The acoustic output sums with the original noise signal in the acoustic far field, thus tending to cancel the noise. In essence, each vibrating surface zone and its associated sound cancellation unit tend to form an acoustic doublet. A signal indicative of the projector output is used as a feedback signal, with appropriate time delays to cancel the effect of the projected output signal being picked up by the unit's transducer, and to cancel the effect of the output of other projectors of the array. Those persons skilled in the art desiring more information concerning this system are referred to U.S. Pat. No. 4,025,724 issued May 24, 1977 to Davidson, Jr. et al.
In another electronic audio noise cancelling device, a microphone is disposed on the outside of an earphone relative to the auditory canal within an ear. An amplifier processing circuit has an input connected to the operational amplifier microphone. The circuit includes a filter channel and an operational amplifier channel. The filter channel includes an active filter and a variable gain amplifier and the operational amplifier channel includes an operational amplifier and a variable gain amplifier. The two variable gain amplifiers are complementary.
The two channels are combined with the summed signal being fed through a user adjustable variable gain amplifier to a summing circuit At the summing circuit a signal on a communication channel is introduced. The summing circuit output is fed to the earphone 12. The amplifier processing circuitry provides for user adjustment of phase and amplification to enable a preferred cancellation signal to be developed to the earphone. The output provides unwanted noise cancellation at the earphone. Those persons having a need for more information concerning the device are referred to UK Patent Application GB 2172769A, published Sept. 24, 1986 for an Ear Defender.
The problems with the above devices stem from acoustic time delay and the use of active filters. In acoustic time delay the distance between the microphone and loudspeaker has a phase shift associated with it; thereby allowing only certain frequencies to be 180 degrees out of phase. In an active filter also, only some frequencies can be made 180 degrees out of phase. Signals that are out of phase by 180 degrees will cancel, but the addition of the phase response of the filter makes for a random response depending on the setting of the filter and the distance between the microphone and loud speaker. An additional problem with the arrangement having a loud speaker downstream of the microphone is that if the speed of sound changes, the frequencies being cancelled also change. Another problem also exists when the microphone is separated from the loud speaker in that a path is created for reflections to enter the ear that are not processed.
Miniature portable sound amplifiers for persons with impaired hearing are well known as hearing aids. A hearing aid includes a microphone, audio amplifier, earphone, and batteries.